Firemen and utility workers respond to a gas explosion and massive fire on Feb. 19, 2013 in the Plaza shopping district in Kansas City, Mo. where a car crashed into a gas main in the upscale shopping district, sparking a massive blaze that engulfed an entire block and caused multiple injuries.

A body was recovered this morning from the scene of a natural-gas explosion that ripped through a popular downtown restaurant in an upscale section of Kansas City, Mo., Tuesday night, leaving 15 people injured.

The body has yet to be identified and three of the injured are in critical condition.

Witnesses say the huge fireball, which engulfed an entire city block Tuesday night, could be seen for miles. The explosion destroyed JJ's restaurant and rattled windows blocks away.

"Everything shook, and you could feel it almost as much as you could hear it," one witness said. "It was by far the biggest explosion I've ever felt in my life."

St. Luke's Hospital said it treated eight patients, including two men who were in critical condition. Six other people had less serious injuries, according to ABC affiliate KMBC-TV. The University of Kansas Hospital said it had received four patients and had a fifth on the way, KMBC reported.

More than 100 firefighters worked overnight to knock back the thick black smoke and shooting flames that forced onlookers to flee the downtown Kansas City shopping district just after 6 p.m.

"We are concerned that there may be additional people not able to get out of the structure," Fire Chief Paul Berardi told reporters late Tuesday. "This was happy-hour at a restaurant. There were patrons in the restaurant.

"I would always fear there are fatalities in a scene like this," he added.

Investigators are sifting through the rubble this morning and trying to figure out what caused the blaze. It is believed that a contractor might have hit an underground natural-gas line.

It almost smelled like the radiators were seeping out gas," one witness said. "Strong enough for us to call the landlord to say something was wrong."

Witnesses say gas fumes could still be smelled hours after firefighters got the flames under control.

On its Facebook page, J.J.'s restaurant, which has been in operation for 27 years said, "Please keep our friends and families in your hearts and prayers. At this time we are still trying to assess the situation and make sure our coworkers and customers are safe."